Field of the Invention
Exemplary embodiments relate generally to compressors and, more specifically, to the provision of thermal barriers for ensuring the smooth operation of a compressor over a wide temperature range.
Description of the Prior Art
A compressor is a machine which increases the pressure of a compressible fluid, e.g., a gas, through the use of mechanical energy. Compressors are used in a number of different applications and in a large number of industrial processes, including power generation, natural gas liquification and other processes. Among the various types of compressors used in such processes and process plants are the so-called centrifugal compressors, in which the mechanical energy operates on gas input to the compressor by way of centrifugal acceleration, for example, by rotating a centrifugal impeller.
Centrifugal compressors can be fitted with a single impeller, i.e., a single stage configuration, or with a plurality of impellers in series, in which case they are frequently referred to as multistage compressors. Each of the stages of a centrifugal compressor typically includes an inlet conduit for gas to be compressed, an impeller which is capable of providing kinetic energy to the input gas and a diffuser which converts the kinetic energy of the gas leaving the impeller into pressure energy.
A multistage compressor 100 is illustrated in FIG. 1. Compressor 100 includes a shaft 120 and a plurality of impellers 130. The shaft 120 and impellers 130 are included in a rotor assembly that is supported through bearings 190 and 190′ and sealed to the outside through sealings 180 and 180′.
The multistage centrifugal compressor 100 operates to take an input process gas from an inlet duct 160, to increase the process gas pressure through operation of the rotor assembly, and to subsequently expel the process gas through an outlet duct 170 at an output pressure which is higher than its input pressure. The process gas may, for example, be any one of carbon dioxide, hydrogen sulfide, butane, methane, ethane, propane, liquefied natural gas, or a combination thereof. Between the impellers 130 and the bearings 190 and 190′, the sealings 180 and 180′ are provided to prevent the process gas from flowing through to the bearings 190, 190′.
Each of the impellers 130 increases the pressure of the process gas. Each of the impellers 130 may be considered to be one stage of the multistage compressor 100. Additional stages, therefore, result in an increase in the ratio of output pressure to input pressure.
Compressors in oil and gas industries and power plants are operated with different gas temperatures. The temperature varies from cryogenic to very high temperature. The internal surfaces in boiled off gas application (BOG) compressors are subjected to cryogenic temperature while the outer surfaces of the compressor are exposed to atmospheric temperature. Due to the cryogenic temperature, thermal contraction occurs in the components. The contraction is not uniform due to variation in temperature on different parts. The non-uniform contraction reduces clearance and/or creates interference between the adjacent components and affects performance of the compressors. In BOG compressors, the differential thermal contraction between the sealings 180 and 180′ (in general, mechanical seals or dry gas seal type or DGS), the end head 140 and 140′ (which could also include heated seal carrier), the bearings 190 and 190′ and the shaft 120 creates interference between them and affects normal operation of the compressor.
In order to remove or reduce thermal tension across the operating temperatures involved, dry gas seals 180 and 180′ are encapsulated in heated seal carriers 140 and 140′ that also act as thermal shields.
It would be desirable to minimize thermal tension and stress on the dry gas seal and the end head by introducing a thermal barrier around the DGS to ensure smooth operation of the BOG compressor.